1. Field of the Invention
The present invention relates to a vulcanizable rubber composition having improved physical properties.
2. Description of the Prior Art
Application fields of rubber composition have been recently broadened and as a result various specific properties have been demanded. For example, a tire, especially a tire used on a rough road, or a rubber article for industry such as a rubber belt for a mine should be prepared by a rubber composition which is not injured by rocks or stones, that is, it should have a high cut growth resistance. According to conventional methods, the cut resistance has been improved by incorporating an inorganic reinforcing agent such as carbon black, silica and the like or an organic reinforcing agent such as phenolic resin, rosin and the like, or incorporating a rubber having a different structure, for example, incorporating an unsaturated polar rubber into an unsaturated non-polar rubber. When the former method of incorporating an inorganic or organic reinforcing agent is used, the cut growth resistance of the vulcanized rubber is improved; however, the resilience of the rubber article decreases and the heat buildup shortens the life of the tire. When the latter method of incorporating an unsaturated polar rubber in an unsaturated non-polar rubber is used, a large quantity of the unsaturated polar rubber is required to improve cut growth resistance of the rubber composition and breaking strength of a vulcanized rubber, while the decrease of resilience can be prevented. As described above, it has been very difficult to obtain a rubber composition having both improved cut resistance and resilience.
On the other hand, cut growth resistance of the rubber composition can be improved by increasing the hardness. A stiffener is used to increase hardness of rubber. As a stiffener for rubber, organic polyamino compounds, quinones, hydrazine sulfate and the like are known. These stiffeners have been used to harden an unvulcanized rubber, prevent the deformation and improve the green strength. However, when these stiffeners are used to improve the hardness of a vulcanized rubber, a large quantity thereof is required. In this case, since vulcanization is greatly accelerated to cause scorching, increase crosslinking density, and cause other properties to deteriorate with increase of hardness. Accordingly the usual stiffeners are not satisfactory.